An error control algorithm can be classified into two types of schemes, i.e., an automatic repeat request (ARQ) scheme and a forward error correction (FEC) scheme.
Examples of the ARQ scheme include a Stop and Wait ARQ scheme, a Go-Back-N ARQ scheme, and a Selective-Repeat ARQ scheme.
The Stop and Wait ARQ scheme is to transmit next frame after acknowledging whether a previous frame has been exactly received (acknowledging through ACK signal). Also, the Go-Back-N ARQ scheme is to transmit N number of successive frames, and to retransmit all the data frames, which are transmitted after an error has occurred, if the N number of successive frames have not been transmitted successfully. The Selective-Repeat ARQ scheme is to selectively retransmit frames only in which an error has occurred.
A Hybrid Automatic Repeat reQuest (HARQ) scheme is to control an error by combining an ARQ scheme with a forward error correction (FEC) scheme, and maximizes error correction coding capability of data received during retransmission.
The HARQ scheme is classified into a chase combining scheme and an incremental redundancy scheme depending on characteristics of bits transmitted during retransmission.
The chase combining scheme (hereinafter, referred to as ‘CC scheme’) is to retransmit data used for first transmission to increase a signal to noise ratio (SNR) of a receiver, thereby obtaining gain.
The incremental redundancy scheme (hereinafter, referred to as ‘IR scheme’) transmits some of parity bits in a different mode during retransmission. In this case, a coding rate can be controlled depending on parity bits which are retransmitted. Since the receiver combines the transmitted data, it obtains coding gain, thereby obtaining improved performance.
The HARQ scheme can be classified into a Synchronous HARQ scheme and an asynchronous HARQ scheme depending on a scheduling scheme of a transmitter.
The transmitter which depends on the synchronous HARQ scheme transmits data using resources, which are previously defined, at the time when both the transmitter and the receiver know. Accordingly, the synchronous HARQ scheme does not need signaling required for transmission, for example, a HARQ process number for identifying data.
Meanwhile, the asynchronous HARQ scheme is to allocate and transmit radio resources for data transmission at a random time. Accordingly, since the asynchronous HARQ scheme includes signaling required for data transmission, for example, a HARQ process number, signaling overhead increases.
The synchronous HARQ scheme and/or the asynchronous HARQ scheme are used in various communication systems. Hereinafter, a 3GPP LTE (Long Term Evolution) system which is an example of the various communication systems will be described.
Table 1 illustrates an example of signaling for supporting HARQ defined in 3GPP TR25.814.
TABLE 1FieldSizeCommentIfHybrid ARQ3Indicates the hybrid ARQasynchronousprocess numberprocess the currenthybrid ARQ istransmission isadoptedaddressing.Redundancy2To support incrementalversionredundancy.New data1To handle soft bufferindicatorclearing.IfRetransmission2Used to derivesynchronoussequenceredundancy version (tohybrid ARQ isnumbersupport incrementaladoptedredundancy) and ‘newdata indicator’ (tohandle soft bufferclearing).
If the asynchronous HARQ scheme is adopted, a HARQ process number of 3 bits, a redundancy version of 2 bits, and a new data indicator of 1 bit are used.
FIG. 1 is a block diagram illustrating signaling of Table 1.
As shown in FIG. 1, if the asynchronous HARQ scheme is adopted, additional signaling is forwarded along with a data block. In other words, three types of signaling marked in Table 1 are added to the data block.
The HARQ process number is information for identifying a HARQ process block where transmission is currently performed. The HARQ process block is a data unit where an error can be detected and transmission and reception are performed in such a manner that information as to whether an error has occurred with respect to information received in the receiver is sent to the transmitter.
The redundancy version of 2 bits is information for identifying a pattern of codewords transmitted when the IR scheme is adopted. As described above, the IR scheme is to retransmit some of the parity bits and information bits constituting codewords. In other words, if retransmission is performed, previous bits and other bits can be retransmitted. Accordingly, if same bits are transmitted in accordance with the IR scheme, since this transmission scheme may be based on the CC scheme, the CC scheme could be referred to as a special IR scheme. In this case, bits used for transmission can be identified by specific index information. The redundancy version is information for identifying bits used for transmission.
The new data indicator of 1 bit is an indicator for identifying retransmission from first transmission. If retransmission is performed, the receiver combines data stored in a buffer with retransmitted data and demodulates the combined data. If first transmission is performed, the receiver can clear a memory stored in the buffer.
Meanwhile, if the synchronous HARQ scheme is adopted, a retransmission sequence number of 2 bits is used. It is possible to support the redundancy version and the new data indicator through the retransmission sequence number.
Hereinafter, features and problems of the aforementioned HARQ scheme according to the related art will be described.
FIG. 2 is a block diagram illustrating action of the ‘Stop-and-wait HARQ’ scheme.
Stop-and-wait HARQ protocol, i.e., Stop-and-wait HARQ scheme is a transmission method which is the simplest and efficient. As shown in FIG. 2, the transmitter transmits one HARQ process block to the receiver. According to the action of the stop-and-wait HARQ scheme, the receiver transmits a HARQ feedback signal to the transmitter, which identifies ACK or NACK, and then the transmitter determines whether to transmit new data or to retransmit current data. However, link transmission efficiency is deteriorated due to a rounding trip time (RTT) required for the transmitter to transmit and receive ACK (ACKnowledgement)/NACK (Negative ACK) to and from the receiver.
FIG. 3 is a block diagram illustrating action of the ‘Stop-and-wait HARQ’ scheme which uses N number of channels.
To improve the related art, the ‘Stop-and-wait HARQ’ scheme which uses N number of channels uses several (N number) independent channels for the time when a transmission link is not used until the ACK/NACK signals are transmitted and received, thereby preventing link transmission efficiency from being deteriorated due to RTT.
According to the scheme of FIG. 3, the transmitter transmits data to the receiver. And, the receiver transmits a HARQ feedback signal to the transmitter, which identifies ACK or NACK, and then the transmitter determines whether to transmit new data or to retransmit the current data. In this case, N number of independent HARQ process channels are operated.
Data for voice over Internet protocol (VoIP), for example, like persistent scheduled data, can be allocated with resources with priority as compared with other data. When the synchronous HARQ scheme is adopted, if data to be transmitted are not transmitted by data such as VoIP at the time when the data should be transmitted, their transmission timing point will be lost. In this case, since latency equivalent to a multiple number of the RTT occurs, system efficiency is deteriorated.